Sodium Blood Test: Low & High Sodium Levels Explained

Approach to Hyponatraemia

Most Common Causes

• SIADH (syndrome of inappropriate ADH): most common in hospital setting (lung cancer, pneumonia, drugs, CNS disease)
• Thiazide diuretics, especially in elderly women
• Heart failure and cirrhosis: dilutional hyponatraemia
• Hypothyroidism: reduces cardiac output and free water excretion
• Polydipsia (excessive water intake): primary or psychogenic

Sodium test: typical price bands across lab types in India

Sodium is rarely ordered alone — it usually travels inside an electrolyte or kidney panel. Typical list-price bands:

These are approximate bands, not quotes: prices change often and vary by city and lab. Where possible, choose a NABL-accredited lab. Sodium is measured by a well-standardised method, so results compare reliably between labs — a genuine change between two reports more likely reflects fluid balance or medication than the assay.

Sodium myths vs facts

Sodium: what matters most

• Read it as a concentration, not a quantity. Low sodium usually means too much water relative to salt — that single reframing makes the whole topic easier to follow.
• The trend and the symptoms outweigh one number. A single borderline value needs confirmation in context — recent IV fluids, medicines and fluid intake all shift it.
• Mention medicines and timing. Thiazide diuretics, SSRIs, SNRIs and carbamazepine can push sodium down; if a low result followed a medicine change, that timing belongs in the conversation.
• Never stop a prescribed medicine over a lab value. The risk of stopping usually outweighs the risk of the number — discuss first.

Frequently Asked Questions

Pharmacist's practical notes

Low sodium is almost always a water-balance problem rather than a salt-deficiency problem. A low result usually means too much water relative to salt, not too little salt in the diet — which is why eating more salt is rarely the treatment and why fluid restriction is so often part of it. Reading sodium as a concentration, not a quantity, makes the whole topic much easier to follow.

Several common medicines can push sodium down, most notably thiazide diuretics, SSRIs and SNRIs, and carbamazepine. If a low sodium appeared soon after starting or changing a medicine, that timing is worth mentioning to your doctor. Never stop a prescribed medicine on your own because of a lab value — the risk of stopping usually outweighs the risk of the number.

A mildly low sodium in someone who feels well is often simply rechecked rather than treated. Because sodium shifts with fluids, medicines and even recent intravenous drips, one unexpected value needs confirmation in context before it means anything. The trend and the symptoms matter far more than a single borderline figure.

Rapid correction of low sodium is genuinely dangerous and belongs in hospital care — the brain injury from correcting too fast can be irreversible. If your result is markedly low, follow your doctor's plan promptly; if it is mildly low and you feel fine, the usual step is a calm recheck, not an emergency salt load at home.

References

Sources cited on this page. PubMed links open the original abstract.

1. Sterns RH. Disorders of plasma sodium, causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65. PMID 25551526 · doi:10.1056/NEJMra1404489
2. Tandukar S, Sterns RH, Rondon-Berrios H. Osmotic Demyelination Syndrome following Correction of Hyponatremia by ≤10 mEq/L per Day. Kidney360. 2021;2(9):1415–1423. PMID 35373113 · doi:10.34067/KID.0004402021
3. Aziz F, Sam R, Lew SQ, et al. Pseudohyponatremia: Mechanism, Diagnosis, Clinical Associations and Management. J Clin Med. 2023;12(12):4076. PMID 37373769 · doi:10.3390/jcm12124076

Sodium measures water, not salt

The most common misunderstanding about this test is that a low sodium means too little salt. In almost all cases it means too much water relative to salt. This is why eating more salt is rarely the treatment for hyponatraemia, and why fluid restriction is so often part of it. Understanding sodium as a concentration rather than a quantity makes the rest of the interpretation considerably easier.

Medications that commonly cause low sodium

• Thiazide diuretics, the single most frequent drug cause, often within the first weeks of starting.
• SSRIs and SNRIs, particularly in older adults, through inappropriate ADH secretion.
• Carbamazepine and oxcarbazepine, and some antipsychotics.
• Proton pump inhibitors and NSAIDs, less commonly but recognised.
• Desmopressin, where the effect is a direct extension of how the drug works.

Why correction speed matters as much as the number

Sodium that has fallen slowly over weeks is tolerated far better than the same value reached in a day, because the brain adapts. That adaptation is also why rapid correction is dangerous: raising sodium too quickly in chronic hyponatraemia can cause osmotic demyelination, a severe and often irreversible neurological injury. For this reason correction is deliberately limited, typically to single figures per 24 hours, and is monitored with frequent repeat testing.² The limit is not a guarantee. A review of published cases found osmotic demyelination occurring even where correction stayed within the European guideline ceiling of 10 mEq/L in 24 hours, almost always in people whose starting sodium was below 115 mEq/L and who had additional risk factors – alcohol use disorder, liver disease, malnutrition or low potassium. In that group the authors argue for holding correction below 8 mEq/L rather than 10.² It is a good example of a laboratory number where the trajectory is managed as carefully as the destination.

High sodium and who it affects

Hypernatraemia is less common and almost always reflects water loss without adequate replacement. It concentrates in people who cannot access or ask for water: infants, those with dementia, people who are sedated or immobile, and anyone with a reduced thirst response. It also occurs with diabetes insipidus and with osmotic losses in uncontrolled diabetes. Because thirst is such a powerful defence, a persistently high sodium in an alert person with free access to fluids is unusual and warrants explanation.